[en] The consequences of Lorentz covariance and of conservation of the nuclear current in calculations of the A_φ asymmetry of the cross sections of (e,e'p) reactions on few-nucleon systems are studied. It is shown that the magnitude of A_φ is very sensitive to various relativistic effects and to the conservation of the nuclear current. In the quasi-elastic region (q²/2mv∼1,p_mis/m much-lt 1), this asymmetry is determined solely by the degree of linear polarization of the virtual photon and by the structure of the electromagnetic current, and at proton emission angles θ_p^c.m>50 degrees the A_φ asymmetry is determined both by the reaction mechanisms and by the intranuclear dynamics. 12 refs., 3 figs